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Nature Shock: Cannibal Hippos

The documentary series examining freak occurrences in the natural world continues. This instalment explores a spate of hippo deaths on a nature reserve in Uganda. Tests confirmed that the hippos were dying from anthrax, possibly contracted from infected soil. However, the unusual nature of the outbreak gave rise to a second, even more shocking theory – that the anthrax was being spread among the hippos through cannibalisation.

In August 2004, the Queen Elizabeth National Park in Uganda was confronted with a most disturbing mystery. After a long spell of dry weather, hippopotamuses began dying in large numbers along the shores of the Kazinga Channel. Carcasses were found bobbing in the water next to live hippos. The initial theory that the deaths were caused by fighting was ruled out when keepers discovered a number of peaceable females amongst the dead. “When the number became bigger is when I became worried,” says keeper Peter Acheroi. “It had never happened before.”

Park wardens theorised that poaching was to blame, until they saw that the ivory teeth of the animals had been left intact. With the death toll rising, chief wildlife vet Dr Patrick Atimnedi travelled to the park to assess the situation. “Why the consistency in mortalities and why these numbers?” he asks. The decomposing corpses posed a risk to human and animal life, and the gruesome spectacle threatened the park’s burgeoning tourist industry.

Vets began to wonder if the deaths were due to an infection. Dr Risto Heinonen, a Finnish vet based in Africa, thought he recognised the symptoms. “These are criteria that fit very well with anthrax,” he says. Anthrax is spread by spores found naturally in patches of earth. Outbreaks occur occasionally in African countries, but they usually involve more than one species of animal. “How is it that it affects only hippos? Where did it come from?” Heinonen asks.

Samples from an autopsy on one of the dead hippos were sent to Germany for analysis. Epidemiologist Fabian Leendertz concluded that the infection was indeed anthrax. It became a race against time to identify the source of the outbreak before more animals became infected. With the government on a state of alert, workers began burying the corpses in the park. Leendertz flew to Uganda to assist in the operation and quickly ruled out his theory that the water was contaminated.

Leendertz now speculated that the anthrax originated in the hippos’ diet. It was most likely caused by hippos eating plants from infected soil. But infected areas are usually quite small, so how was it that hippos all over the park were dying? The answer came when Leendertz noticed unusual behaviour in the hippos still living in the Kazinga Channel. “Around the carcass there were always other hippos… and they were really interested in the carcass,” he says. “That was a bit suspicious for me.” Then he saw hippos chewing on the carcasses’ intestines, which had been expelled from the body by the gases inside.

Leendertz’s theory held that, with the dry weather killing off vegetation, the hippos had been forced to seek other food sources – including their own kind. But the notion was met with derision – not least because hippos never eat meat. “We have never seen a hippo eating meat, we know that hippos are vegetarian,” says Nicholas Kauta, head of the clean-up operation.

In search of further evidence, Leendertz read an article by a Dr Joseph Dudley, who had witnessed a hippo eating an impala in Zimbabwe. Then footage came to light of a hippo in Malawi eating a hippo carcass. The astonishing theory now seemed irrefutable – hunger-stricken hippos in Uganda were eating the infected corpses of their brethren, only to become infected with anthrax themselves. This explained the spread of the outbreak and why it was restricted to one species. “The hippopotamus then becomes the world’s largest omnivore,” says Dr Dudley, “and, as far as I can tell, the only such mega-omnivore in the past 20 million years.”